Antennas are used in a variety of applications for transmission and receipt of information via electromagnetic waves. The direction at which an antenna radiates or receives power can be optimized by the shape and structure of the antenna, as well as the method of driving it. In some applications, a highly directional antenna is desired, while in others an omnidirectional antenna is desired. In the transmission mode, an input signal connects to a feed on the antenna and drives a radiator. The electrical signal of the input is converted to electromagnetic radiation that propagates from the radiator in accordance with its directivity. The process basically works in reverse when an antenna is receiving a signal.
In addition, for maximum efficiency, the load presented by the antenna itself, or more specifically, by the radiator of the antenna, should be matched to the input impedance of the feed. This minimizes loss due to reflections and standing waves created by impedance mismatching.
Space considerations also play a role in antenna design. For example, an elongated antenna (such as a traditional dipole) may provide an ideal power distribution pattern for a given application; however, the device or product of which the antenna is a part, or the application in which the antenna is used, may not permit the use of a long, somewhat fragile antenna such as a traditional dipole.
For terrestrially based applications, in which the device receiving signals from or transmitting to an antenna is positioned away from the antenna at relatively small angle from horizontal, it is desirable that the antenna's power distribution be directed primarily outward (or horizontally), rather than vertically. A traditional dipole antenna provides such a radiation pattern but often proves too large or fragile for a given application. One use of antennas includes transmitting from a location located at or near ground level to receivers located on power or telephone poles, or buildings, which may be located in any direction from the antenna. In such locations, the size of the antenna is a key consideration, as well as the likelihood that the antenna will inevitably come into contact with persons or objects.
When a dipole, ring, yagi, or similar type antenna is fed with a coax connection, the coaxial cable may act as a radiator, in addition to the radiator of the antenna itself. To isolate the antenna radiator from the coax feed cable, and prevent coax cable from radiating, a choke balun may be added between the antenna and the feed line. This is prior art. These types of antennas, however, do not have a ground plane. Some circular antennas include a ground plane having concentric circular grooves formed in it, effectively leaving a series of concentric circular walls. In these devices, the choke is “above” the ground plane, with respect to the feed line.
For antennas with a radiator positioned over a ground plane, such as a patch antenna, prior art designs assume that that the ground plane isolates the radiator from the feed line (which is connected from below the ground plane), such that the feed line does not affect or interfere with the radiation pattern of the antenna. It has been discovered, however, that the ground plane does not provide adequate isolation and a coaxial feed cable can interfere with radiation patterns of antenna, even where the antenna radiator is separated from the coaxial cable by the ground plane.
Thus, there is a need for a relatively compact antenna that provides a substantially omnidirectional power distribution oriented primarily horizontally, rather than vertically. There is also a need for an antenna that is structurally resistant to bumps and knocks that may be experienced in a terrestrial installation. There is also a need for further isolating the radiation patterns of an antenna in which the radiator is separated from a feed, such as coaxial feed line, by a ground plane.